It is always desirable to reduce the cost of manufacturing pneumatic tires especially those intended for passenger vehicle use even a slight amount, due to the large number of tires produced and the resulting overall savings achieved thereby. However, it is necessary to maintain and preferably improve the ride characteristics and performance of the tire or maintain the present status thereof while reducing the manufacturing costs.
One of the main elements of a pneumatic radial ply tire construction is the body carcass which generally is made of organic fiber, e.g., polyamide, polyester, Nylon or the like. Heretofore, the body carcass of the majority of passenger tires use twisted yarn having a degree of twist generally in the area of 10 or more twists per inch. A plurality of these twisted yarns then were intertwined to form a twisted cord which formed the body ply. These twisted cords then are arranged in a generally parallel relationship with respect to each other and are interconnected by transversely extending cross threads referred to as wefts or picks. Although this construction forms a strong body carcass, it increased the cost due to the large amount of material which is required to produce the high twist yarns and ultimate twist cords. Also, the high twist cords and weft strands provide nonuniformity to the carcass body which may affect the ride achieved by the final tire construction.
Another problem present in passenger vehicle tires, especially in radial tires by virtue of the flexibility in the side walls to provide a soft comfortable ride, is that the flexibility of the side walls tends to reduce the lateral stability and cornering property of such tires. The cyclic loading that is experienced by the side wall of the tire during one revolution or footprint of the tire results in compression and tension loadings in the turn-up region of the side wall and body ply cords therein providing an area of cyclic strain or oscillation in the side wall. This strain can result in fatigue of the ply cords and possible subsequent separation in the side wall area. This problem is amplified where the turn-up ends of the body carcass or ply terminate in the cyclic or oscillation zone of the side wall.
The importance of the location of the turn-up ends of the body ply has been recognized in the prior art and the problems encountered therewith by its loosening from the side wall while providing sufficient length to insure anchoring of the ply on the steel bead core wires, and that the length of the turn-up ends effects the overall tire characteristic. For example, U.S. Pat. No. 3,774,663 attempts to solve this problem by inserting additional plies extending about the bead core to provide longer and higher turn-up ends instead of continuing the main body ply cords to the longer desired termination point, and in which the connecting body extensions include rubber having a modulus of elasticity higher than the rubber connecting the cords of the main carcass body.
The construction of U.S. Pat. No. 4,231,409 attempts to eliminate this problem of fatigue in the side wall of a radial tire due to the difference in stiffness existing between the side wall and the material surrounding the bead wire core, as well as the effect of the turn-up distance of the carcass body ply ends by the addition of two additional layers of textile cords located in the side wall area where the relatively large cyclical load or oscillation occurs, especially when the tire rotates at high speeds. In the tire construction of this patent the turn-up ends extend to about 15% of the tire section height and the bead filler has a Shore hardness of 90.degree. which extends to 18% of the section height.
This excessive flexing problem in the side wall of radial tires also was recognized in U.S. Pat. No. 3,253,639 which attempts to solve the problem by restricting the height of the turn-up ends of the body ply to a height of between 8% and 25% of the tire section height in combination with a reinforcing ply terminating below the top of the turn-up ends in the bead area. This problem of the turn-up body ply ends in the area of flexing stress and oscillation also is recognized in U.S. Pat. No. 4,265,292 which attempts to solve the problem by providing a steel chafer which is located outside of the turn-up ends in combination with a particular type of organic fiber material located outside of the steel chafer, with the turn-up ends of the main body ply terminating below the height of the steel chafer.
U.S. Pat. No. 3,682,223 shows a tire construction which attempts to eliminate the stresses in a radial tire, especially in the bead area, by providing a bead filler of a relatively hard material having a hardness of between 75 and 95 durometer, in combination with a pair of cushioning rubber strips adjacent the main bead filler of various hardness less than that of the main bead filler. In this construction the turned-up ends of the main body ply extends beyond the bead filler strip.
U.S. Pat. No. 3,841,378 also attempts to provide a solution to the excess stress occurring in the bead region of a tire adjacent the steel rim by providing a reinforcement of rubberized fleece material fibers in this area. U.S. Pat. No. 4,261,405 is of some interest in respect to the present invention in that it discloses a tire in which the turn-up end extends a substantial distance along the side wall of the tire and includes a bead filler between the turn-up ends of the body ply and the main body ply in the side wall area. However, the body ply is formed of stranded cord material and not low twist yarn. U.S. Pat. No. 4,202,393 discloses another tire which attempts to provide increased durability to the tire by reducing the stress in the side wall and bead area by providing a bead filler located within a separate side wall reinforcing layer of cord material.
The main incentive to utilize low twist yarn in the body ply for tires is for cost reduction. As the twist level of yarn is decreased, textile manufacturing steps are shortened or eliminated. Changes in twist level also alter the geometry and mechanical behavior of the filament bundle. The main effects of using reduced twists in cords and yarns have been found to be a higher modulus, lower elongation, higher breaking strength, lower fatigue and abrasive resistance, and lower adhesion pullout force from a substrate.
Low twist cords and yarns have been tested as direct replacements for conventional tire body cords (10.times.10-12.times.12) in an attempt to achieve the advantage of the low twist yarn. However, the performance of these tires was unsatisfactory due to the lower side wall flex breaks that initiated within the turn-up ply. These failures were attributed to the inherent reduced fatigue resistance of low twist cords and yarns.
Analysis of tires that failed from lower side wall flex breaks showed that the flex break began as filament structure of the turn-up ply in the lower side wall due to compressional fatigue followed by crack growth through the turn-up and bead filler and finally crack propagation through the sidewall. Thus, a standard tire has to be redesigned in order to reduce the increased fatigue that is inherent in a tire using a low twist yarn.
Body ply turn-up undergoes cyclic loading as the tire goes in and out of the footprint and experiences lateral loads (i.e., cornering). This cyclic loading action is undesirable for the cords in a tire when the load cycle goes from tension to compression. This condition can be tolerated within limits with high twist cords. However, it is generally unacceptable for low twist yarns in the range of between 3 and 4 turns-per-inch. Because of the processing and material cost savings afforded by low twist yarns, a tire construction able to use such low twist yarn while providing the same resistance and durability to such cyclic loading as provided by body ply using high-twist cords is desirable.
The use of a low twist yarn and cords in the construction of pneumatic tires has been known in the art as shown in U.S. Pat. Nos. 2,273,200 and 2,309,564. However, the particular tire construction with which these low twist yarns or cords are used are completely different from that of the present invention, which provides a combination of elements that enables low twist yarn to be used in radial ply tires by overcoming the inherent lower fatigue resistance of such yarn.
Furthermore, in addition to developing a tire using a body ply formed of low twist yarn, it is desirable that the tire body ply be comprised only of parallel aligned cords or twisted yarn without the connecting pick or weft threads since it achieves increased uniformity. Therefore, it is desirable to develop a tire having a particular side wall design in which the stresses in the turn-up ply can be reduced or redistributed when using low twist yarn since tests show that tire durability decreases to unacceptable levels as body cord twist level decreases, and that the majority of failures are due to lower side wall flex breaks that initiated within the turn-up ply. Thus there is a need for a tire design, the parameters of which reduce or redistribute compressional forces in the lower turn-up region of the tire, and which have applicability to production building methods. Although the prior art has recognized the problems attempted to be solved by the following disclosure, the various combinations and arrangements of the ply material, bead filler hardness, height, and width which are set forth herein and which achieves the desired advantages, are not shown, suggested or obvious in view of the known prior art.